Load maximum 1mb blob data for a diff file

Currently, it's defaulted to 10mb which is unnecessary
because the blobs with a size greater than 1mb
are not displayed

app/models/blob.rb

@@ -83,9 +83,9 @@ class Blob < SimpleDelegator
     new(blob, project)
   end
 
-  def self.lazy(project, commit_id, path)
+  def self.lazy(project, commit_id, path, blob_size_limit: Gitlab::Git::Blob::MAX_DATA_DISPLAY_SIZE)
     BatchLoader.for([commit_id, path]).batch(key: project.repository) do |items, loader, args|
-      args[:key].blobs_at(items).each do |blob|
+      args[:key].blobs_at(items, blob_size_limit: blob_size_limit).each do |blob|
         loader.call([blob.commit_id, blob.path], blob) if blob
       end
     end

app/models/repository.rb

@@ -516,10 +516,12 @@ class Repository
   end
 
   # items is an Array like: [[oid, path], [oid1, path1]]
-  def blobs_at(items)
+  def blobs_at(items, blob_size_limit: Gitlab::Git::Blob::MAX_DATA_DISPLAY_SIZE)
     return [] unless exists?
 
-    raw_repository.batch_blobs(items).map { |blob| Blob.decorate(blob, project) }
+    raw_repository.batch_blobs(items, blob_size_limit: blob_size_limit).map do |blob|
+      Blob.decorate(blob, project)
+    end
   end
 
   def root_ref

changelogs/unreleased/id-limit-blob-data-for-diffs.yml

+---
+title: Load maximum 1mb blob data for a diff file
+merge_request: 24160
+author:
+type: performance

lib/gitlab/diff/file.rb

@@ -350,6 +350,16 @@ module Gitlab
 
       private
 
+      def fetch_blob(sha, path)
+        return unless sha
+
+        # Load only patch_hard_limit_bytes number of bytes for the blob
+        # Because otherwise, it is too large to be displayed
+        Blob.lazy(
+          repository.project, sha, path,
+            blob_size_limit: Gitlab::Git::Diff.patch_hard_limit_bytes)
+      end
+
       def total_blob_lines(blob)
         @total_lines ||= begin
           line_count = blob.lines.size
@@ -385,15 +395,11 @@ module Gitlab
       end
 
       def new_blob_lazy
-        return unless new_content_sha
-
-        Blob.lazy(repository.project, new_content_sha, file_path)
+        fetch_blob(new_content_sha, file_path)
       end
 
       def old_blob_lazy
-        return unless old_content_sha
-
-        Blob.lazy(repository.project, old_content_sha, old_path)
+        fetch_blob(old_content_sha, old_path)
       end
 
       def simple_viewer_class

spec/lib/gitlab/diff/file_spec.rb

@@ -169,18 +169,18 @@ describe Gitlab::Diff::File do
     end
   end
 
-  describe '#old_blob' do
-    it 'returns blob of commit of base commit' do
-      old_data = diff_file.old_blob.data
+  describe '#old_blob and #new_blob' do
+    it 'returns blob of base commit and the new commit' do
+      items = [
+        [diff_file.new_content_sha, diff_file.new_path], [diff_file.old_content_sha, diff_file.old_path]
+      ]
 
-      expect(old_data).to include('raise "System commands must be given as an array of strings"')
-    end
-  end
+      expect(project.repository).to receive(:blobs_at).with(items, blob_size_limit: 100 * 1024).and_call_original
 
-  describe '#new_blob' do
-    it 'returns blob of new commit' do
+      old_data = diff_file.old_blob.data
       data = diff_file.new_blob.data
 
+      expect(old_data).to include('raise "System commands must be given as an array of strings"')
       expect(data).to include('raise RuntimeError, "System commands must be given as an array of strings"')
     end
   end

spec/models/blob_spec.rb

@@ -22,6 +22,7 @@ describe Blob do
     let(:same_project) { Project.find(project.id) }
     let(:other_project) { create(:project, :repository) }
     let(:commit_id) { 'e63f41fe459e62e1228fcef60d7189127aeba95a' }
+    let(:blob_size_limit) { 10 * 1024 * 1024 }
 
     it 'does not fetch blobs when none are accessed' do
       expect(project.repository).not_to receive(:blobs_at)
@@ -30,7 +31,9 @@ describe Blob do
     end
 
     it 'fetches all blobs for the same repository when one is accessed' do
-      expect(project.repository).to receive(:blobs_at).with([[commit_id, 'CHANGELOG'], [commit_id, 'CONTRIBUTING.md']]).once.and_call_original
+      expect(project.repository).to receive(:blobs_at)
+        .with([[commit_id, 'CHANGELOG'], [commit_id, 'CONTRIBUTING.md']], blob_size_limit: blob_size_limit)
+        .once.and_call_original
       expect(other_project.repository).not_to receive(:blobs_at)
 
       changelog = described_class.lazy(project, commit_id, 'CHANGELOG')
@@ -53,7 +56,8 @@ describe Blob do
 
       readme = described_class.lazy(project, commit_id, 'README.md')
 
-      expect(project.repository).to receive(:blobs_at).with([[commit_id, 'README.md']]).once.and_call_original
+      expect(project.repository).to receive(:blobs_at)
+        .with([[commit_id, 'README.md']], blob_size_limit: blob_size_limit).once.and_call_original
 
       readme.id
     end